1. Field of the Invention
The present invention relates to a method and to an apparatus for preventing the occurrence of impact pressure in an injection molding machine, in order to prevent impact which would otherwise be induced in an injection step by a hydraulic pressure which builds upon the start of injection.
2. Description of the Relevant Art
Conventionally, there are known two screw driving systems for injection molding machines; i.e., a hydraulic drive system and a motor drive system.
As disclosed in Japanese Patent Application Laid-Open (kokai) No. 8 (1996)-25450, in a conventional injection molding machine which employs a hydraulic drive system, a servo valve is changed over upon the start of injection in an injection step so as to supply pressure oil from a hydraulic oil source to the rear oil chamber of an injection cylinder. The supplied pressure oil causes a screw to advance from an injection start position, thereby injecting a measured amount of resin into and filling a mold. Oil contained in the front oil chamber of the injection cylinder returns to an oil tank through an oil transfer line.
Upon the start of the injection, high-pressure oil is supplied into the rear oil chamber of the injection cylinder in an impactive manner, and thus associated impact acts on oil contained in the oil transfer line via a piston, causing a so-called water hammer phenomenon. This water hammer phenomenon is substantially of a negligible degree for ordinary injection molding machines. However, for injection molding machines having a high injection speed, the degree of water hammer cannot be disregarded. Specifically, water hammer becomes obvious in the form of a considerably loud noise (impact sound) or significant vibration, and associated impact pressure has an adverse effect on a hydraulic circuit and hydraulic devices.